JP5955342B2 - Humidity sensor comprising a polymer layer containing a mixture of polyamides as a moisture absorbing layer - Google Patents

Description

  The present invention relates to a humidity sensor having a high water absorption rate, sufficient sensitivity between 0 and 100% relative humidity, and sufficient stability over time of this sensitivity.

The humidity sensor of the present invention can find application in many fields where humidity control is essential, among others:
-Agricultural fields where a certain degree of humidity (especially in the greenhouse) can be important for crop growth;
-The field of food production where excessive humidity can increase the rate of decaying stocks and the growth of molds;
-The pharmaceutical sector, where the water content of powder-type drugs must be particularly tightly controlled;
-Electronics (electronics) field where it is essential to be able to control humidity in a clean room and to detect the presence of leaks in enclosed electronic components;
-The ceramic manufacturing industry where the humidity must be controlled in relation to the dry part before firing, or the paper industry, or the industry that involves the use of gases for chemical synthesis (here excessive moisture in these gases) Fields of the chemical industry, such as content can lead to changes in the chemical composition of the product;
-The field of transport, such as automobiles, aviation, household appliances (especially tumble dryers) and the field of mass consumer goods;
The medical field, for example in exhaled humidity management or in the detection of leaks in blood processing fluid systems;
-The field of art preservation where it is important to manage the humidity of facilities where art works are stored (eg museum rooms) to prevent them from deteriorating.

  The use, detection and / or measurement of humidity in many of the above fields (known in the scientific term “humidity measurement method”) is the subject of many studies and analyses, which are humidity measurement devices (hygrometers or humidity sensors). And more specifically the ratio between the relative humidity [the amount of water vapor contained in a given medium at a given temperature and the maximum amount it can contain (equal to the saturation point) And this ratio is expressed as an RH value that can vary from 0 to 100%.

In order to be effective and reliable, the hygrometer should ideally meet the following list of specifications:
An extremely short reaction time (eg less than 2 minutes);
Linear response as a function of -0 to 100% relative humidity (RH);
-Low hysteresis;
-Low temperature coefficient;
A wide operating temperature range, preferably -20 ° C to 80 ° C;
-Stability over time of the measurement (especially after the hygrometer has been stored in a low or high humidity ambient atmosphere and / or in aggressive media)

  Hygrometers may use a wide variety of parameters, with the variety of physical phenomena in which water vapor plays some role, and each parameter provides a specific humidity measurement means.

  Among the most commonly used hygrometers, hygrometers based on the principle of direct humidity measurement (in the case of condensation hygrometers) are based on the principle of measuring the properties of objects related to humidity (variable impedance ( in the case of variable-impedance).

  Condensation hygrometers actually consist of measuring the dew point of air (i.e., the temperature at which humid air saturates or, in other words, the temperature at which water vapor contains liquid condensation). In order to achieve this, a sample of air for which it is desired to determine the relative humidity must be taken, and this must be measured, for example, in contact with a cooled mirror. . When this temperature reaches the dew point, the moisture present in the air condenses and the beam of light hitting the mirror can no longer be reflected or reflected at a changed reflection angle. By measuring the temperature at which the beam is interrupted or changed, the dew point temperature and further the relative humidity of the air can be obtained.

  In principle, this type of hygrometer has a long reaction time. Because the temperature sensor present in the hygrometer generally does not come into direct contact with the mirror (which must measure its temperature), which sometimes requires waiting for a few minutes before obtaining the mirror temperature limit. It means that.

  With respect to variable impedance hygrometers, they are based on the use of sensitive elements including hygroscopic materials, and variations in their electrical properties are measured by environmental humidity.

  Resistive hygrometers are distinguished from capacitive hygrometers depending on whether the electrical property whose variation is measured is electrical resistance or capacitance.

  In the case of a resistance hygrometer, a certain amount of hygroscopic material is deposited on the substrate in a pattern that constitutes a resistor. Considering that the two electrodes are connected to this pattern and thus the resistance between the electrodes depends on the water content and the temperature, thereby reducing the electrical resistance as the amount of water increases. The moisture content of the medium in which the hygrometer is placed can be obtained.

  Finally, in the case of capacitive hygrometers, a dielectric layer capable of absorbing ambient humidity is located between the two electrodes, thereby forming a capacitor. When the humidity changes, the amount of water absorbed by this layer also changes, which changes the dielectric constant of this layer, resulting in a change in the capacitance of the capacitor, which is measured to determine the measured capacitance value. Makes it possible to obtain the moisture content of the environment in which the capacitive hygrometer is located.

The dielectric used may be a porous inorganic material, for example porous silicon, alumina Al ₂ O ₃ , silica SiO ₂ , but the use of such materials is due to the phenomenon of capillary condensation. This results in a considerable hysteresis phenomenon, usually resulting in a relative humidity of 55-97% RH in the case of inorganic materials having a pore size of 2-50 nm. Thus, they would be advantageously used only for low humidity measurements (eg, relative humidity below 20% RH).

  The dielectric materials used may also be organic materials, in particular polymeric materials, which are usually more advantageous with regard to response linearity and hysteresis phenomena than with inorganic materials.

  These polymeric materials are usually selected for their ability to constitute a low absorptivity dielectric material, so that water vapor becomes easier to desorb when passing from a high relative humidity environment to a low relative humidity environment Meeting these criteria and conventional materials belong to the family of cellulose acetate butyrates, polyimides, polysulfones or polyether sulfones.

However, these materials have certain disadvantages, among which can be mentioned:
Low sensitivity to low humidity values (eg 0-5% RH) in the case of capacitive hygrometers with cellulose acetate butatorates, polyimides, polysulfones or polyether sulfones as dielectric material ;
A very long stabilization time in the case of cellulose acetate butyrates and polyimides;
-Substantial hysteresis phenomenon after 30 minutes waiting time of each humidity measurement in increasing or decreasing direction for relative humidity above 8% when the dielectric used comprises cellulose acetate butyrate or polyimides.

Therefore, there is a need for a humidity sensor (or hygrometer) with a polymer absorbent material that can overcome the above disadvantages, and in particular may have the following advantages:
-Stability of sensitivity over time;
-High sensitivity greater than the sensitivity of sensors with the use of polymers such as polyimides, cellulose acetate butyrate and polyethersulfones [thus measuring very low rates of humidity (eg 0-5% relative humidity) And allows for more accurate measurements over the 0-100% relative humidity range];
An extremely short reaction time;
-Low hysteresis.

  Accordingly, the present invention is a humidity sensor comprising a polymer layer comprising a polymer mixture comprising a first polyamide and a second polyamide as a moisture-absorbing layer, wherein the second polyamide is a repeat thereof. It relates to a humidity sensor which contains more carbon atoms in the unit than in the first polyamide repeating unit.

  Before describing in more detail in the description of the invention, the inventors define the following definitions.

  Repeat unit is understood to mean a divalent group derived from one component after its polycondensation.

  Polyamide is a polycondensation that includes an amide group —C (═O) —NH— that is usually generated from the polycondensation step between the amine functional group of the compound constituting these functional groups and the carboxylic acid in its main chain. It is understood to mean a thing.

Thus, a mixture of other polyamides (the second polyamide having more carbon atoms in its repeating unit than the first polyamide, specifically more —CH ₂ — groups than the first polyamide. By producing a sensor having a moisture-absorbing layer as a moisture-absorbing layer, the inventors of the present invention can obtain such a first polyamide or such a second It is surprising that it has greater sensitivity and stability over time than a sensor with only a polyamide in the moisture-absorbing layer, and especially reduces or eliminates crystallization phenomena that can cause irreversible degradation of sensor sensitivity. It was possible to prove by the method.

  Without being bound by theory, the stability of the sensitivity of the sensor of the present invention is that it does not crystallize under the influence of humidity, as in the case of a sensor having only a single polyamide in its absorbing layer. This can be explained by the stability of the ratio of the crystal zone to the sex zone. Thus, the absorption rate of the sensor of the present invention proves to be stable even after temperature and humidity cycles, or even after long periods of high or low humidity. In addition, the use of polyamide mixtures as described above provides the absorbent layer with a plasticity that remains stable over time, thereby experiencing a polymer or mixture of polymers that tend to crystallize and cure over time. The risk of crushing is reduced.

  More specifically, the polymer layer may consist solely of polyamide.

  According to the present invention, the first polyamide can be selected from among polyamide 6, polyamide 6-6 and polyamide 11, and the second polyamide is selected from polyamide 6-6, polyamide 6-10 and polyamide 12. Note that, according to the above conditions, the first polyamide is selected from the above list, so that its repeating units have fewer carbon atoms than the second polyamide. To do.

The following are recalled:
-Polyamide 6 is a polyamide generally obtained by polycondensation of caproic acid and hexamethylene diamine resulting in a polyamide comprising an array of repeating units having the following formula:

  -Polyamide 6-6 is a polyamide generally obtained by polycondensation of adipic acid and hexamethylene diamine resulting in a polyamide containing an array of repeating units having the formula:

  -Polyamide 6-10 is a polyamide generally obtained by polycondensation of sebacic acid and hexamethylenediamine, resulting in a polyamide comprising an array of repeat units having the formula:

  -Polyamide 11 is a polyamide generally obtained by polycondensation of aminoundecanoic acid, resulting in a polyamide comprising an array of repeat units having the following formula:

  -Polyamide 12 is a polyamide generally obtained by polycondensation of aminododecanoic acid, resulting in a polyamide comprising an array of repeating units having the formula:

More specifically, the polymer layer may comprise a mixture selected from the following mixtures:
A mixture of polyamide 6 and polyamide 6-6;
A mixture of polyamide 6 and polyamide 6-10;
A mixture of polyamide 6-6 and polyamide 6-10;
A mixture of polyamide 11 and polyamide 12,
And preferably
-Mixtures of polyamide 6 and polyamide 6-6 (for example 50/50 mass ratio).

  The mixture that can be used in the description of the invention may have a mass ratio of the first polyamide to the second polyamide in the range of 95/5 to 5/95, for example 50/50, Also good.

  In addition to the presence of the first polyamide and the second polyamide, the polymer layer may comprise other polyamides, for example a third polyamide, a fourth polyamide and / or a fifth polyamide.

  The sensor of the present invention may be a capacitive sensor or a resistance sensor, in which case it may comprise at least one polymer layer as described above, located between the first electrode and the second electrode. Here, the first electrode and the second electrode may be in contact with the same substrate.

Such a support may be made of glass or, optionally, from a semiconductor material, for example silicon covered with an electrical insulator layer made of silicon dioxide SiO ₂ , in order to reduce leakage. It may be made.

  The first electrode deposited on the possible support may be made of a conductive material, for example a metallic material such as tantalum.

  The second electrode is also advantageously partly a conductive material, for example a metallic material, and the surrounding moisture moves to such second electrode and comes into contact with the polymer layer. It is advantageous to have porosity so that this is possible. This conductive material may take the form of a multilayer of metal layers such as a chromium layer, a nickel layer and a gold layer.

  In such sensors, humidity can be detected by measuring variations in sensor capacitance or resistance.

  In fact, moisture in contact with the polymer layer changes its dielectric constant, resulting in variations in sensor capacitance (which can be associated with a relative humidity value equal to the humidity of the environment in which the sensor is located). .

  The sensor of the present invention may also take the form of a sensor comprising a beam or film completely or partially covered by a polymer layer as described above.

Such sensor detection of humidity can be achieved by:
Measuring the fluctuations of the resonant frequency of the powder or film, where such fluctuations depend on the amount of moisture absorbed (in this case, the sensor can be considered a cMUT (Capacitive Micromachined Ultrasonics) type sensor) it can);
Measuring surface waves caused by absorption of moisture by the polymer layer; or absorption of moisture by the polymer layer (this absorption results in plasticization of the material of the layer and changes in the curve of the beam or film. Resulting in the deformation of the powder or film caused by, for example, piezoresistance, capacitance or optical methods.

  The sensor of the present invention finds application in all fields where it is necessary to detect the presence of moisture or to know the amount of humidity present in the environment, which can be found in the agricultural, food manufacturing and pharmaceutical fields. Applicable to electronics field, chemical industry field, transportation and mass consumer goods field, medical field and art preservation field.

  More particularly, with respect to the electronics field, the sensor of the present invention detects leaks in components enclosed in a vacuum or nitrogen, such as accelerometers, gyroscopes, or pressure-type sensors. These sensors may be used, where the sensors are typically made of silicon sealed with resin beads, soluble glass beads, or beads made of eutectic alloy, or an integral cover made of polysilicon. Is generally protected by. In the case of this type of component, the sensor of the present invention can detect leakage from 1 ppm. This is because the humidity (typically 20-40% relative humidity) contained in the ambient atmosphere of the temperature / humidity component installation space diffuses in the moisture absorption layer of the sensor as soon as leakage occurs. It is.

  The invention has been described in connection with the above embodiments, which are presented by way of illustration and not limitation.

1 shows a capacitive humidity sensor of the present invention. It relates to a sensor according to the invention which is provided for a change in relative humidity from 11% to 97% RH (curve a) and for a change in relative humidity from 97% to 11% RH (curve b). It is a graph which shows change of deviation D as a function of time t (seconds). FIG. 6 is a graph showing the change in frequency F as a function of time t (number of hours) for a sensor of the present invention that is subjected to cycling with respect to relative humidity as illustrated in this figure (where this cycling is Application of cycle A as described in FIG. Changes in hysteresis variation as a function of time t (months) for sensors made of polyamide 6 or polyamide 6-6 subjected to a specific cycle as described below (curves H1 and H2 respectively) ) (Positive vertical axis) and change in sensitivity S as a function of time t (months) (as a percentage) (curve S) (negative vertical axis).

Embodiments of Humidity Sensors of the Invention A mixture of polyamides was prepared from 1 g of polyamide 6 and 1 g of polyamide 6-6 dissolved in 40 cm ³ of formic acid. This mixture is then deposited on an anodized tantalum electrode belonging to the humidity sensor according to invention 1 shown in the attached FIG. 1 by spin coating at 3,000 rpm. Each of these sensors includes:
A glass substrate 3 having the following dimensions (50 mm * 50 mm);
The anodized tantalum electrode 5 (referred to as the first electrode), a 250 nm thick electrode deposited through a first stencil on one side of the substrate;
A contact electrode 7 comprising an overlay of a chromium layer (20 nm thickness), a nickel layer (200 nm thickness) and a gold layer (100 nm thickness) (also deposited on this substrate through a second stencil).

  The mixed layer (No. 9 in FIG. 1) deposited in this manner is subjected to a heat treatment on a hot plate at 80 ° C. for 2 minutes, thereby removing the organic solvent, followed by vacuum annealing at 250 ° C. for 1 hour, Cool quickly inside.

  A 20 nm thick porous chromium layer (number 11 in FIG. 1) is then applied by vacuum evaporation (vacuum evaporation) at a rate of 1 nm / second after mechanical etching of the polymer layer in the contact electrode 7. Deposited through stencils. Here, mechanical etching was carried out with a steel tool in order to remove the polymer layer by peeling.

  This porous chromium layer contacts the contact electrode 7 through the holes present in the polymer layer, where the assembly formed by this layer and this contact electrode forms the second electrode 13.

  As a variant, it can be envisaged to deposit a porous chromium layer on the first electrode and the contact electrode at the same time, resulting in two vertical capacitors mounted in series.

  The sensor obtained by the above method (forming a vertical capacitor) is subjected to a humidity test.

  To do this, a sensor is inserted into the oscillator so that the capacitance variation of the capacitor can be measured when the capacitor is exposed to changes in humidity.

  By these measurements of capacitor capacitance variations (when the sensor is inserted in an oscillator with a fundamental frequency of 100 kHz), for relative humidity varying from 11% to 97% RH, and from 97% to 11 It is shown that very rapid response times are obtained for relative humidity changing to% RH.

  In fact, for a change in relative humidity from 11% to 97% RH (curve a) and for a change in relative humidity from 97% to 11% RH (curve b) as a function of time t (expressed in seconds). As shown by the results reported in FIG. 2 which shows the change in deviation D, 98% of the deviation is less than 2 minutes in the course of the above two humidity changes (more strictly speaking, 11% to 97% 1 minute 43 seconds for the change to RH and 1 minute 55 seconds for the change from 97% to 11% RH).

The sensor described above is also subjected to the following cycle (hereinafter referred to as cycle A) as shown in FIG.
-1 hour at 11% relative humidity;
-1 hour at 75% relative humidity;
-1 hour at 97% relative humidity;
-1 hour at 75% relative humidity;
-1 hour at 11% relative humidity;
-16 hours at 97% relative humidity;
-1 hour at -75% relative humidity; followed by 1 hour at -11% relative humidity,
The reciprocal of capacity (ie frequency) is measured over the course of this cycle.

The above cycle is used to characterize the most important parameters to be considered when comparing humidity sensors.
a) Short-term hysteresis H ₁ (this is the difference between the values of frequency F obtained at 75% relative humidity before and after a period of constant conditions of 1 hour at 97% relative humidity, ie:
H ₁ = F ₄ -F ₂ = 0.5%
b) Maximum hysteresis H ₂ (this is the difference in value at 75% relative humidity before and after a period of constant conditions of 16 hours at 97% relative humidity):
_{H 2 = F 7 -F 2 =} 3%
c) Sensitivity S (which is the frequency F obtained between the first value at 75% relative humidity and the value obtained after a constant condition period of 16 hours at 97% relative humidity. Value and the first value obtained after 1 hour at 11% RH):
_{S = (F 6 -F 1)} / ΔHRmax = (F 6 -F 1) / (97-11)

  It can be observed that the sensor according to the invention has a low hysteresis value over the course of this cycle.

  With regard to sensitivity, the sensor of the present invention has a sensitivity that is 10% greater than the sensitivity of sensors whose sensitive elements are made solely of polyamide 6 or polyamide 6-6.

At the same time, a sensor according to the invention identical to the sensor described above was subjected to cycling according to cycle A described above after 1, 3 and 9 months of cycling in the environmental chamber according to the following cycle (referred to as cycle B). :
-Overnight at 10 ° C at 30% relative humidity;
-Overnight at 20 ° C at 45% relative humidity;
-Overnight at 60 ° C at 90% relative humidity;
-Overnight at 20 ° C at 45% relative humidity;
Here, the reciprocal of the capacity is measured during the cycle A.

  It can be observed that after cycling in the environmental chamber, the sensitivity (determined based on the equation shown above) remains constant after 1, 3 and 9 months of cycling with cycle B.

  For similar sensors made of polyamide 6 or polyamide 6-6, FIG. 4 [positive vertical axis represents the change in hysteresis (H1 and H2 respectively) as a function of time (months) and negative The vertical axis represents the change in sensitivity S (as a percentage) as a function of time (in months)], after cycling for 1 month, 3 months and 9 months in the environmental chamber by cycle B, respectively. Sensitivity is reduced by 3%, 5% and 10%.

Given the value of hysteresis, they increase regularly over time. Ie:
* After one month of cycling by cycle B, H1 is equal to 2% and H2 is equal to 4%;
* After 3 months of cycling with cycle B, H1 is equal to 3%, H2 is equal to 6%; and
* After 9 months of cycling by cycle B, H1 is equal to 4% and H2 is equal to 8%.

Claims (8)

A humidity sensor comprising a polymer layer comprising a mixture comprising a first polyamide and a second polyamide as a moisture-absorbing layer, wherein the second polyamide is a repeating unit of the first polyamide in its repeating unit. look contains a number of carbon atoms than the unit, said first polyamide, polyamide 6, selected from among polyamide 6-6 and polyamide 11, wherein the second polyamide, polyamide 6-6, polyamide 6-10 And a humidity sensor selected from polyamide 12 .   The humidity sensor according to claim 1, wherein the polymer layer is made of only polyamide. The humidity sensor according to claim 1 or 2 , wherein the polymer layer comprises the following mixture:
A mixture of polyamide 6 and polyamide 6-6;
A mixture of polyamide 6 and polyamide 6-10;
A mixture of polyamide 6-6 and polyamide 6-10;
A mixture of polyamide 11 and polyamide 12;
A humidity sensor comprising a mixture selected from. The humidity sensor according to any one of claims 1 to 3 , wherein the polymer layer includes a mixture of polyamide 6 and polyamide 6-6. In said mixture, said first polyamide, relative to the second polyamide is present in a weight ratio of between 95/5 and 5/95, according to any one of claims 1-4 Humidity sensor. Comprising at least one defined polymer layers in any one of claims 1 to 5, wherein the layer is located between the first electrode and the second electrode, any one of claims 1 to 5 The humidity sensor described in 1. The humidity sensor according to claim 6 , wherein the first electrode and the second electrode are in contact with the same substrate. The polymer layer according to any one of claims 1 to 5, and a fully or partially covered by the beam or membrane, a humidity sensor according to any one of claims 1-5.

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